Rebulid when the source has been updated

You could use src/*.rs src/*/*.rs src/*/*/*.rs,
but explicit listing is prefered.

Ideally, you should be separate object files that
are rebuilt only when necessary, and then build
the final binary from those. But have not looked
at how to compile rust code so I don't know how
to do that, and since there are no header files
that is also probably suboptimal.

Signed-off-by: Mattias Andrée <maandree@kth.se>

Makefile

@@ -1,3 +1,33 @@
+SRC = \
+	src/info/sources.rs \
+	src/info/mod.rs \
+	src/info/filetype.rs \
+	src/bin/main.rs \
+	src/term.rs \
+	src/exa.rs \
+	src/output/grid_details.rs \
+	src/output/tree.rs \
+	src/output/colours.rs \
+	src/output/grid.rs \
+	src/output/cell.rs \
+	src/output/mod.rs \
+	src/output/details.rs \
+	src/output/lines.rs \
+	src/output/column.rs \
+	src/fs/file.rs \
+	src/fs/fields.rs \
+	src/fs/mod.rs \
+	src/fs/dir.rs \
+	src/fs/feature/xattr.rs \
+	src/fs/feature/git.rs \
+	src/fs/feature/mod.rs \
+	src/options/misfire.rs \
+	src/options/filter.rs \
+	src/options/dir_action.rs \
+	src/options/view.rs \
+	src/options/mod.rs \
+	src/options/help.rs
+
 PREFIX = /usr/local
 
 CARGOFLAGS = --no-default-features
@@ -8,7 +38,7 @@ build: CARGOFLAGS=
 build: all
 build-no-git: all
 
-target/release/exa:
+target/release/exa: $(SRC)
 	if test -n "$$(echo "$$CC" | cut -d \  -f 1)"; then \
 	    env CC="$$(echo "$$CC" | cut -d \  -f 1)" cargo build --release $(CARGOFLAGS); \
 	else\